Configure the option "Workspace color" (set the color; File | Options)

Here you can configure the color that Presentations uses to paint the area outside the page. Note that you must also set the option to use a user-defined color.

WSColorRGBValue

Registry Hive	HKEY_LOCAL_MACHINE or HKEY_CURRENT_USER
Registry Path	SOFTWARE\Policies\SoftMaker\Presentations 2018
Value Name	WSColorRGBValue
Value Type	REG_DWORD
Default Value	8421504
Min Value	0
Max Value	16777215